A Nano Tool Chest
Imagine putting some wires, transistors, and other electronic components into a bag, shaking it, and pulling out a radio-fully assembled and ready to work. Although this sounds fanciful, such remarkable “self-assembly” is, in essence, what chemists do whenever they synthesize materials. Mixing solutions in a beaker, a chemist lets the intrinsic attractions and repulsions of certain molecules and atoms take over. An art and science has evolved to arrange conditions so that atoms spontaneously assemble into particular molecular structures.
Similarly, we are surrounded and inspired by products that are marvelously complex and yet very inexpensive. Potatoes, for example, consist of tens of thousands of genes and proteins and intricate molecular machinery; yet we think nothing of eating this miracle of biology, mashed with a little butter. Potatoes, along with many other agricultural products, cost less than a dollar a pound. The key reason: if provided with a little soil, water, air, and sunlight, a potato can make more potatoes. Likewise, if we could make a general-purpose programmable manufacturing device that was able to make copies of itself-what nanotechnology researchers call an assembler-then the manufacturing costs for both the device and anything it made could be kept low.
A basic principle in self-assembly is selective “stickiness.” If two molecular parts have complementary shapes and charge patterns-that is, one has a hollow where the other has a bump, or one has a positive charge where the other has a negative charge-then they will tend to stick together in a particular way to form a bigger part. This bigger part can combine in the same way with other parts so that a complex whole emerges from molecular pieces.
Self-assembly is not by itself sufficient, however, to make the wide range of products that nanotechnology promises. If the parts are indiscriminately sticky, for example, then stirring them together would yield messy blobs instead of precise molecular machines. We can solve this problem by holding the molecular parts in the proper position and orientation so that when they touch they will join together the way we want them to. At the macroscopic scale, the idea that we can hold parts in our hands and assemble them by properly positioning them with respect to each other goes back to prehistory: we celebrate ourselves as the tool-using species. But the idea of holding and positioning molecules is new and almost shocking. Nanoscale equivalents of “arms” and “hands” must be developed.
Current proposals for molecular-scale positional devices resemble normal-sized robotic devices, but they are about one ten-millionth as big. A molecular robotic arm could sweep systematically back and forth, adding and withdrawing atoms from a surface to build any structure that the computer instructed it to. Such an arm, composed of a few million atoms, might be 100 nanometers long and 30 nanometers around. Although it would have roughly 100 moving parts, it would use no lubricants-at this scale, a lubricant molecule is more like a piece of grit. Such ultraminiature tools should be able to position their tips to within a small fraction of an atomic diameter. Trillions of such devices would occupy little more than a few cubic millimeters (a speck slightly larger than a pinhead).
Molecular arms would be buffeted by something we don’t worry about at the macroscopic scale: thermal noise. Atoms and molecules are in a constant state of wiggle and jiggle; the higher the temperature, the more vigorous the motion. To maintain its position, therefore, a nanoscale arm must be extremely stiff.
The stiffest material around is diamond. The strength and lightness of a material depends on the number and strength of the bonds that hold its atoms together, and on the lightness of the atoms. The element that best fits both criteria is carbon, which is lightweight and forms stronger bonds than any other atom. The carbon-carbon bond is especially strong; each carbon atom can bond to four neighboring atoms. In diamond, then, a dense network of strong bonds creates a strong, light, and stiff material. Indeed, just as we named the Stone Age, the Bronze Age, and the Steel Age after the materials that humans could make, we might call the new technological epoch we are entering the Diamond Age.
How can a diamond device of this scale be produced? One answer comes from looking at how we grow diamond today. In a process somewhat reminiscent of spray painting, we build up layer after layer of diamond by holding a surface in a cloud of reactive hydrogen atoms and hydrocarbon molecules. When these molecules bump into the surface they change it, either by adding, removing, or rearranging atoms. By carefully controlling the pressure, temperature, and the exact composition of the gas in this process, called chemical vapor deposition (CVD), we can create conditions that favor the growth of diamond on the surface.
But randomly bombarding a surface with reactive molecules does not offer fine control over the growth process; it is akin to trying to build a wristwatch using a sand blaster. We want the chemical reactions to occur at precisely the places on the surface that we specify. A second problem is how to make the diamond surface reactive at the particular spots where we want to add another atom or molecule. A diamond surface is normally covered with a layer of hydrogen atoms. Without this layer, the raw diamond surface would be highly reactive because it would be studded with the carbon atoms’ unused (or “dangling”) bonds. While hydrogenation prevents unwanted reactions, it also renders the entire surface inert, making it difficult to add carbon (or anything else) to it.
To overcome this problem, we could use a set of molecular-scale tools that would, in a series of steps, prepare the surface and create structures on the layer of diamond, atom by atom and molecule by molecule. The first step in the process would be to remove a hydrogen atom from a specific spot on the diamond surface, leaving behind a reactive dangling bond. This can be done with a “hydrogen abstraction tool”-a molecular structure that has a high chemical affinity for hydrogen at one end but is elsewhere inert. The tool’s unreactive region serves as a kind of handle. The tool would be held by a molecular positional device, such as the molecular robotic arm discussed earlier, and moved directly over particular hydrogen atoms on the surface we wish to abstract.
This creates a chicken-and-egg problem: we need a molecular robotic arm to build another molecular robotic arm. To solve this problem, we must at some point build a molecular robotic arm with something other than a molecular robotic arm. We could, for example, use a macroscopic positional device-such as an improved version of an existing atomic-force microscope-to make our first molecular robotic arm. Alternatively, we could self-assemble a simplified molecular positional device. These first crude positional devices could then be used to make better ones.
One suitable molecule for a hydrogen abstraction tool is the acetylene radical-two carbon atoms triple bonded together. One carbon would be the handle, and would link to a nanoscale positioning tool. The other carbon has a dangling bond where a hydrogen atom would be in ordinary acetylene. The environment around the tool would be inert (typical proposals involve the use of either vacuum or a noble gas, such as krypton or xenon).
Once this tool has created a reactive spot by selectively removing hydrogen atoms from the diamond surface, it becomes possible to deposit carbon atoms at the desired sites. In this way a diamond structure is built, molecule by molecule, according to plan. One proposal for this function is the dimer deposition tool. A dimer is a molecule consisting of two of the same atoms or molecules stuck together. In this case, the dimer would be C2-two carbon atoms connected by a triple bond. In the deposition tool, each carbon in the dimer would be connected to a larger molecule by single bonds with oxygen atoms.
The hydrogen abstraction tool and dimer deposition tool would work together (see illustration above). First, the abstraction tool would remove two adjacent hydrogen atoms from the diamond surface. The two dangling bonds would react with the ends of the carbon dimer. This reaction would break the carbon-oxygen bonds and then transfer the carbon dimer from the tool to the surface. Because the energy released during the reaction is much larger than thermal noise, the dimer will “snap” onto the surface and stay there.
A third proposed tool for making nanostructures is the carbene insertion tool. Carbenes-highly reactive carbon atoms with two dangling bonds-will react with (and add a carbon atom to) many molecular structures. Carbenes will readily insert into double or triple bonds, like the bond in the carbon-carbon dimer described above. A positionally controlled carbene could be attached almost anywhere on a growing molecular workpiece, leading to the construction of virtually any desired shape.
A fourth proposal is for a hydrogen deposition tool. Where the hydrogen abstraction tool is intended to make an inert structure reactive by creating a dangling bond, the hydrogen deposition tool would do the opposite: make a reactive structure inert by terminating dangling bonds. Such a tool would let us stabilize reactive surfaces and prevent the surface atoms from rearranging in unexpected and undesired ways. The key requirement for such a tool is that it include a weakly attached hydrogen atom. While many molecules fit that description, the bond between hydrogen and tin is especially weak; thus, a tin-based hydrogen deposition tool should be effective.
These four molecular tools should enable us to make a wide range of stiff structures-but only those that are composed of hydrogen and carbon. This is a much less ambitious goal than attempting to use all 100 or so elements in the periodic table. But in exchange for confining ourselves to this more limited class of structures, we make it much easier to analyze those that can be fabricated and the synthetic reactions needed to make them. In any case, this narrower proposal can be more readily and more thoroughly investigated than full nanotechnology. And diamond and its shatterproof variants fall within this category, as do the “fullerenes”-sheets of carbon atoms rolled into spheres, tubes, and other shapes. These materials can compose all the parts needed for basic mechanical devices such as struts, bearings, gears, and robotic arms.
Ultimately we’d like to add other elements-to create diamond electronic devices, for example, or add some nitrogen to the internal surface of a bearing in order to relieve strain (the carbon-nitrogen bond is longer than the carbon-carbon bond). Such structures, composed primarily of carbon and hydrogen in combination with nitrogen, oxygen, fluorine, silicon, phosphorous, sulfur, or chlorine, constitute what we call the class of “diamondoid” materials.